The windmill has developed over its long history into many different and varied forms. These varied forms seek to expound upon particular aerodynamic or economic factors. In some cases, prior art windmills have been designed for aerodynamic efficiency in low speed winds. Such high efficiency windmills often were prone to damage or destruction from excessive rotational speed when exposed to high speed winds. Since the velocity of winds vary over a wide range in most areas, aerodynamic efficiency is not the sole factor for providing an economically attrative windmill.
All windmills suffer from the large forces developed on the windmill and its tower or other supporting structure during times of high velocity winds. These large forces require that both the windmill and supporting structure be strongly built. Windmills are usually mounted atop relatively tall towers to take advantage of the higher velocity winds which exist only a short distance above the ground. High wind loads applied to the windmill and tower create very large bending moments which the tower must support. In the case of horizontal axis windmills, these large bending moments cannot be alleviated by the use of guy wires or struts because the rotating windmill blades pass near to the tower along all sides of the tower as the direction of the winds change and the windmill turns with the wind. This dictates that long, slender towers without guy wires be used. Such towers are relatively expensive and the overall cost of the windmill installation is increased when compared to towers supported by guy wires.
Large scale prior art horizontal windmills also cause additional forces to be applied to the tower when the large rotor blades pass very near the windmill tower. In such large windmills, the periodic passage of the blades by the tower can create a vibratory loading which further accentuates the problem of building a sufficiently strong tower.
Another problem suffered by prior art vertical and horizontal windmills is the large fluctuations in windmill rotor speed caused by varying wind velocity. This variation in the rotor speed creates difficulties in coordinating power generated by windmills into an electrical distribution system having a fixed frequency of alternating current. The great fluctuation in windmill speed also creates problems in the mechanical design of windmills since stresses increase considerably with increased rotational speed.
Sudden changes in wind direction also have been a problem in obtaining optimum efficiency from windmills. This is especially true with horizontal axis windmills which are resistant to changes in wind direction because of the gyroscopic effect of the spinning windmill. This gyroscopic effect reduces the efficiency of the windmill because it cannot change direction sufficiently fast so as to take advantage of the full force of the wind. Vertical axis windmills have been effective in dealing with the wind direction change and gyroscopic effect problems but have still suffered from wide fluctuations in angular speed due to wind velocity changes. Sudden changes in wind direction can also cause the rotor blades to warp or flap and in some cases damage or destroy them.
The current intention has been directed to solving the problems discussed above and other problems using the windmill structure described below.